Lupin the brave, Sirius the terrorist?

[dicentra63]

No: HPFGUIDX 41971

Pippin observes: 
>   If Lupin is irrational with fear for Harry, there's no sign of it
> once he reaches the Shack. He's pale and tense, but quite in 
> control of himself. The first thing he does is confiscate the Trio's 
> wands; an odd way of trying to protect them, it seems to me. The 
> next thing he does is ask about Pettigrew. In fact, it's Pettigrew 
> that seems to concern him the whole time.
> 
>  The only time we see Lupin in anything like a panic is when 
> Sirius is about to kill Peter before he can give an account of 
> himself.  Then he yells, "NO!" in All Caps, mind you, and flings 
> himself bodily on Sirius, further injuring Ron. And when Ron tries 
> to leave, it's Lupin who stops him, at wand point. Strange, since 
> he supposedly ran out to the Shack to save Ron's life.
> 

Ooooh, Pippin.  More LYCANTHROPE (aka Evil!Lupin), eh?  Lupin's going
out of his way to protect his fellow DE?  Not so concerned about the
Trio, right?

Ok. So maybe that's not what you're insinuating, but your observation
is interesting nonetheless.  We do assume that Lupin is motivated by
concern for the Trio's safety when he dashes out of the castle.  But
maybe he really is more interested in Peter.  Maybe he has reason to
believe that all the people involved, including Sirius and Peter, are
at risk.

Just like everyone else, Lupin has believed that Sirius is after
Harry.  But by the time he sees the events outside the Whomping
Willow, he has two reasons to think that the dynamic between the Trio,
Sirius, and Peter is different from what it seems.

First, he knows that Sirius turned up at Ron's bedside, not Harry's.
Even to the casual reader this event is a bit of a red flag: something
is not right.  Lupin knows Sirius and knows he's not stupid.  He had
to wonder if maybe he wasn't going after Harry after all.

Second, he suspects that Harry "knows" that Sirius betrayed his
parents.  When he tells Harry about the Dementor's Kiss, Harry blurts
out that Sirius "deserves it."  "You think so," said Lupin, lightly.
"Do you really think anyone deserves that?"  Lupin appears to be
trying to draw Harry out, to assess his knowledge of the situation
(his "light" tone indicating that he's operating on more than one
level).  Harry declines to tell him what he knows, but Lupin has to
suspect that someone has told him the whole story of his parents'
betrayal.

So when he sees Sirius drag Ron and Peter into the Whomping Willow
(when he could have just as easily taken Harry), he knows that Sirius
is not in fact after Harry -- he's after Peter, and he's probably
going to kill him.  Furthermore, he sees Harry and Hermione go after
Sirius and Peter.  He must think it possible that Harry will kill
Sirius as well, if not for betraying his parents then for taking Ron. 

Whatever the dynamic of the siutation, he knows that it's volatile and
that there's a very good chance someone will be wrongfully killed.  He
decides to intervene.

When he arrives, there's Harry pointing his wand at a bleeding and
battered Sirius, but there's no sign of Peter.  Evidently to him, he
was right about Harry's motives.  He disarms the Trio to prevent harm
to Sirius and to generally defuse the situation, then asks about
Peter.  After Sirius's response, he puts two and two together as only
HE can and realizes that Sirius is not a danger to the Trio.  But he's
not going to allow Sirius to kill Peter until he knows exactly what
the deal is, to prevent Peter's wrongful death.  It's only when he's
satisfied that Peter is indeed guilty that he rolls up his sleeves and
takes aim.

So yeah, it appears that when Lupin runs to the Shrieking Shack, it's
not because he thinks Sirius is about to kill Harry, it's because he
thinks Sirius is about to kill Peter, perhaps wrongfully.  And he's
likely worried that Harry might take revenge on the wrong person.

--Dicey, who still likes LYCANTHROPE because it's gloriously creepy